Can current velocity mediate trophic cascades in a mountain stream

Summary Trophic cascades can be context-dependent in that they occur under some conditions but not others. An important context for stream benthic communities is near-bed current velocity, which varies at the spatial scale at which benthic species interactions occur. To investigate how centimetre-scale variation in near-bed current velocity influenced the occurrence of benthic cascades, I studied a stonefly/mayfly/algal system in a high-gradient Rocky Mountain stream and examined the effect of 10 and 25 cm s−1 current (i.e. ‘slow’ and ‘fast’, respectively) on stonefly predation and its top-down effects. There were two experiments using stream-side mesocosms and a third using in-stream troughs. The two mesocosm experiments had known numbers of stoneflies and mayflies and examined how near-bed current influenced (i) top-down cascades driven by non-trophic, behaviourally mediated predator effects (stonefly mouthparts were glued shut) and (ii) direct predation. The in-stream experiment allowed mayfly grazers to arrive and depart freely and examined how near-bed current and stoneflies influenced algal accumulation and community structure. Results from these experiments showed that top-down cascades occurred in slow, but not fast current, and that these cascades are driven largely by the non-trophic effects of predation. The mesocosm experiment showed that mayflies removed less algae in slow current when stoneflies with glued mouthparts were present. There was 33% greater algal biomass in slow compared with fast current, and algal biomass in the slow current did not differ from that in the grazer-free controls. The predation experiment showed that stoneflies captured approximately twice as many mayflies in slow as compared to fast current. The in-stream experiment found that algal biomass increased when stoneflies were present, but grazer densities did not differ from stonefly-free controls, supporting the hypothesis that effects mediated by behaviour drove the cascade. This study suggests that near-bed current can turn stonefly/mayfly/algae cascades ‘on’ and ‘off’ at small spatial scales and provides a new perspective for understanding algal heterogeneity on natural streambeds.